Electrically heatable filaments



Aug. 10, 1965 A. R. BAKER ELEGTRICALLY HEATABLE FILAMENTS Original Filed Feb. 5, 1959 Z In 01%: mff w United States Patent 3,200,011 ELECTRICALLY HEATABLE FILAMENTS :Alan Richard Baker, Shefiield, England, assignor to Na- This invention relates to electrically heatable filaments for use in connection with the detection of combustible gas in a combustion supporting atmosphere, for which purpose it is usual to employ a pair of filaments constituting detecting and compensating elements, respectively, arranged in a Wheatstone bridge circuit. This known arrangement could be such that oxidation of the combustible gas, for example by the oxygen contained in a sample of air under test, would take place at the surface of one of these filaments thus to alter its resistance, but such oxidation and change of resistance at the other filament would be inhibited. The detector filament in such an arrangement is therefore capable of not only producing sufficient heat to promote the said oxidation but of varying its own resistance with change of heat resuiting from oxidation of combustible gas.

In apparatus of the above type, the detecting filaments are often heated to a temperature of 900 C. or more, depending upon the particular metal of which the filaments are made. At these relatively high temperatures, metal evaporates from the surface of the filament, in consequence of which, the resistance of the filament is continually increasing, especially during the oxidation of combustible gases. This continuous increase in resistance, which is greater in the detecting filament than in the compensating filament, causes an increase in the zero reading of the bridge circuit which latter must therefore be reset at frequent intervals.

The present application is a divisional application in respect of certain features originally included in United States patent application Serial No. 790,935, and now Patent No. 3,092,799, which last mentioned application relates to an electrically heatable device for use in the detection of combustible gases in air.

The object of the present invention is to provide a method of manufacturing a heatable device such as described in the specification forming part of United States patent application Serial No. 790,935.

According to the present invention, the method of making the said electrically heatable device for the detection of combustible gas in a combustion supporting atmosphere by the utilisation of the change of temperature of an electric heating filament, includes the steps of coating the surface of the filament with a refractory material and adding to this refractory material a catalyst. This catalyst, when heated to a suitable temperature by the filament will promote oxidation of the gas on the surface of the catalyst so that inward conduction of heat from this oxidation will take place through the said refractory coating to raise the temperature of the filament. Preferably, although not necessarily, the further step is included of adding to the exterior of the coated filament a further coating of refractory material, and the device may be heated to cause some of the catalyst to diffuse through said further coating to the outer surface thereof. The purpose of this further coating will be hereinafter explained.

The invention will now be described, by way of example, with reference to the accompanying drawing.

Referring now to the said drawing, a filament 57 which may be of metal of the platinum group and in the form of a helically coiled wire, is embedded within or com- "ice pietely surrounded by an oxide or other refractory material, which may for example be alumina or silica, so as to form a coating or pellet 58. The alumina or silica is applied to the filament by causing evaporation and decomposition by the application of heat to a paste or solution of a compound such as a nitrate, hydroxide or alkali silicate. More particularly, the alumina pellet may be formed by the application to the coiled filament 57 of successive layers or portions of a saturated solution of aluminum nitrate in water, and after the application of each layer or portion, heating the layers or portions by passing an electric current through the filament.

When the detector device is to be used in apparatus for the detection of combustible gas in air, such as described, for example, in my co-pending United States patent application Serial No. 790,870, and now Patent No. 3,117,843, a catalyst such as palladium or a mixture of platinum and palladium may be added which, by impregnation tends to form an outer coating 59 surrounding the oxide pellet 58 of the filament constitutting the detecting element. This catalyst may be applied as a solution or dispersion of a compound or compounds of one or more metals of the platinum group. One example of catalyst solution which may be used, is obtained by mixing:

1.2 ml. of 8 percent (by weight) palladous chloride (PdCl in 6 N hydrochloric acid,

0.8 m1. of 5 percent (by weight) of chloroplatinic acid (H PtCl l-I O) in water 0.2 ml. of 10 percent (by weight) of aluminum nitrate (Al(NO .6H O) in water.

The liquid is removed and the catalyst made active by heating in air, or if necessary, in another gas.

Alternatively the catalyst may be condensed on the pellet 53 by heating either the internal filament 57, or an external filament in close proximity to the pellet, to a temperature above 1000 C. In so far as the compensating filament is concerned this may comprise a similarly embedded filament to the refractory material surrounding which no catalyst has been added. When the apparatus has to be used with the filaments operating at high temperature, there is tendency for small quantities of the filament material to diffuse into the refractory surrounding these filaments, which diffused material may, give rise to catalytic action, so that, in the case of the compensating filament, it is desirable to add to the refractory coating of this, a suitable metallic oxide such as chromium oxide or some other catalyst poison.

According to a modification adapted more particularly for use as a detecting element, and having its filament embedded with an oxide or other refractory material such as alumina or silica coated externally with a catalyst in the manner already described, the pellet 58 is provided with an additional coating of alumina or other refractory material applied on the top of the external coating of catalyst 59. After heating to approximately 1000 C. for a few minutes, prefreably in a combustible gas, enough of the catalyst 59 diffuses through the additional coating to the external surface of the latter. The effect of this coating is to reduce the loss in catalytic activity which occurs when the detecting element is heated for several hours in air or in low concentrations of methane.

When the filaments are to be used in apparatus for the detection of one non-combustible gas in the presence of another or others, or high concentrations of a combustible gas in air such as described, for example, in co-pending United States patent application Serial No. 790,870, two similar filaments may be used, to both of which a catalyst poison has been added.

Where a detecting device according to the invention is used, the amount of oxidation of the combustible gas at the filament wire is very considerably reduced and the filament can be operated at a lower temperature than would be the case should an exposed filament be used, as a result of which the rate of evaporation of metal from the wire filament is negligible. The necessity, therefore, to make frequent zero adjustments to the electrical bridge as required by the use of an exposed filament in gas detection apparatus as hitherto employed, no longer arises.

In addition, the operational life of the embedded filament is considerably longer than is the case with exposed filaments. The refractory coating, in effect, also adds considerable strength to the filament.

I claim:

1. The method of making an electrically heatable device for the detection of combustible gas in air by the utilisation of the change of electrical resistance of an electric heating filament, including firstly the step of coating the filament with a refractory material, secondly the step of adding to the refractory material a catalyst, thirdly the step of addingto the exterior of the filament a further coating of refractory material, and finally the step of heating the device to cause some of the catalyst to dilfuse through said further coating to the outer surface thereof.

2. In the method of making an electrically heatable detecting device as claimed in claim 1, the final heating step being performed at approximately 1000 C. in an environment of combustible gas.

3. The method of making an electrically heatable device for detection of combustible gas in air by the utilisation of the change of electrical resistance of an electric heating filament, including firstly the step of coating the filament with a refractory material and secondly the step of adding to the refractory material a catalyst solution obtained by mixing:

1.2 ml. of 8 percent (by Weight) of palladous chloride (PdCl in 6, N hydrochloric acid,

0.8 ml, of 5 percent (by Weight) of chloroplatinic acid (H Pt Cl .6H O) in Water,

0.2 ml. of 10 percent (by weight) of aluminum nitrate (A-l(NO .6H O) in water;

thirdly the step of heating to remove the liquid and to render the catalyst active, fo-urthly the step of adding to the exterior of the coated filament a further coating of refractory material, and finally the step of heating the device to cause some of the catalyst to diffuse through 'said further coating to the outer surface thereof.

References Cited by the Examiner UNITED STATES PATENTS RICHARD D. NEVIUS, Primary Examiner. 

1. THE METHOD OF MAKING AN ELECTRICALLY HEATABLE DEVICE FOR THE DETECTION OF COMBUSTIBLE GAS IN AIR BY THE UTILISATION OF THE CHANGE OF ELECTRICAL RESISTANCE OF AN ELECTRIC HEATING FILAMENT, INCLUDING FIRSTLY THE STEP OF COATING THE FILAMENT WITH A REFRACTORY MATERIAL, SECONDLY THE STEP OF ADDING TO THE REFRACTORY MATERIAL A CATALYST, THRIDLY THE STEP OF ADDING TO THE EXTERIOR OF THE FILAMENT A FURTHER COATING OF REFRACTORY MATERIAL, AND FINALLY THE STEP OF HEATING THE DEVICE TO CAUSE SOME OF THE CATALYST TO DIFFUSE THROUGH SAID FURTHER COATING TO THE OUTER SURFACE THEREOF. 